The present invention relates to organic electroluminescent devices. More particularly, it relates to using new cathode materials.
Organic electroluminescent (EL) devices or organic light-emitting diodes (OLEDs) are electronic devices that emit light in response to an applied potential. The structure of an OLED comprises, in sequence, an anode, an organic EL medium, and a cathode. The organic EL medium disposed between the anode and the cathode is commonly comprised of an organic hole-transporting layer (HTL) and an organic electron-transporting layer (ETL). Holes and electrons recombine and emit light in the ETL near the interface at the HTL. Tang et al. xe2x80x9cOrganic Electroluminescent Diodesxe2x80x9d, Applied Physics Letters, 51, 913 (1987), and commonly assigned U.S. Pat. No. 4,769,292 demonstrated highly efficient OLEDs using such a layer structure. Since then, numerous OLEDs with alternative layer structures have been disclosed. For example, there are three-layer OLEDs that contain an organic light-emitting layer (LEL) between the HTL and the ETL, such as that disclosed by Tang et al. xe2x80x9cElectroluminescence of Doped Organic Thin Filmsxe2x80x9d, Journal Applied Physics, 65, 3610 (1989). The LEL commonly consists of a host material doped with a guest material, wherein the layer structures are denoted as HTL/LEL/ETL. Further, there are multilayer OLEDs that contain a hole-injecting layer (HIL), and/or an electron-injecting layer (EIL), and/or a hole-blocking layer, and/or an electron-blocking layer in the devices. These structures have further resulted in improved device performance.
Similar to the organic layer structures in the OLEDs, cathode also plays an important role in the overall performance of OLEDs. A cathode in the OLED is expected to have not only good electron-injecting ability, but also good corrosion resistance. As is known, the commonly used Mg:Ag cathode (Tang and VanSlyke, U.S. Pat. No. 4,885,211) and LiF/Al cathode (Hung and Tang, U.S. Pat. No. 5,776,622) have good electron-injecting ability, but they are subject to corrosion under ambient conditions. Moisture is detrimental to the cathode of OLEDs. Although encapsulation is applied to OLEDs for moisture prevention, it is difficult to fully get rid of moisture after encapsulation, and cathode corrosion at the edge and on the top of each light-emitting pixels still exists, which forms dark edge and dark spots to reduce effective emitting area. Especially for the OLEDs on flexible polymer substrate, moisture elimination is even more difficult to deal with. Therefore, cathode with better corrosion resistance is necessary for stability improvement of OLEDs.
Some metal alloys are used as cathode in OLEDs for the purpose of improving adhesion property or improving electron injection ability, such as the aforementioned Mg:Ag, Al:Mg (VanSlyke and Tang, U.S. Pat. No. 5,059,862), Al:Li (Hung, et al. U.S. Pat. No. 6,140,763), and Al:Li:Cu:Mg:Zr (Nakaya et al. U.S. Pat. No. 6,172,458). Those metal alloys contain at least one kind of low work function metal (by low means the value of work function is less than 4.0 eV) and are still subject to corrosion under ambient conditions.
It is an object of the present invention to provide an OLED with a high work function metal alloy cathode having enhanced corrosion resistance.
It is another object of the present invention to provide an OLED with low dark edge growth rate.
It is another object of the present invention to improve the operational stability of OLEDs.
These objects are achieved in an organic electroluminescent device comprising, in sequence:
a) an anode;
b) an electroluminescent medium; and
c) a metal alloy cathode comprising at least two metals, wherein all the metals of the alloy have their work function higher than 4.0 eV.
An advantage of the present invention is that a high work function metal alloy cathode can be made useful in OLEDs to enhance the corrosion resistance and to reduce the dark edge growth rate of the emitting area in the OLEDs.
Another advantage of the present invention is that the operational stability of the OLEDs can also be improved by using a high work function metal alloy cathode adjacent to an electron-injecting layer in the OLEDs.